Thyroid function testing before conception and during pregnancy

Routine screening for thyroid dysfunction in women who are pregnant or planning to become pregnant is not recommended.

Screening is only recommended in women at increased risk of a thyroid disorder, including women with:

• a history of any thyroid disorder
• a family history of any thyroid disorder
• type 1 diabetes or another autoimmune disorder
• prior or current positive thyroid antibodies (eg euthyroid Hashimoto thyroiditis)
• a history of infertility, recurrent miscarriage or preterm delivery
• a goitre on examination
• prior therapeutic head or neck irradiation.

In women with risk factors for a thyroid disorder, test thyroid function at the first antenatal visit, and consider repeating the test every 4 to 6 weeks during the first trimester to monitor the need for treatment. If the serum TSH concentration is normal, testing again at around 30 weeks is sufficient. In addition, women with a history of Graves disease require specific monitoring to detect neonatal hyperthyroidism.

To avoid misinterpretation of thyroid function test results during pregnancy, use population-based, trimester-specific reference ranges. Refer to local reference ranges supplied by the laboratory; if these are not available, reasonable trimester-specific reference ranges for serum thyroid stimulating hormone (TSH) concentration are shown in Trimester-specific reference ranges for serum TSH concentration during pregnancy.

During a healthy pregnancy, production of thyroid hormones (triiodothyronine [T₃] and thyroxine [T₄]) increases by up to 50%, stimulated by human chorionic gonadotrophin (hCG) in the first trimester. TSH is transiently suppressed by the elevated T₃ and T₄ concentrations. Although total T₄ increases in pregnancy, pregnancy-related changes in binding protein concentrations can cause standard laboratory tests to show a reduced serum free T₄ during the third trimester.